Semiconductor device including field-effect transistor using salicide (self-aligned silicide) structure and method of fabricating the same

ABSTRACT

An element isolation region for electrically isolating an element region where an element is to be formed is formed in a semiconductor substrate. A gate insulating film is formed on the semiconductor substrate in the element region. A gate electrode is formed on the gate insulating film. Source/drain regions are formed to be separated from each other in a surface region of the semiconductor substrate. The source/drain regions sandwich a channel region formed below the gate insulating film. Gate sidewall films are formed on the two side surfaces of the gate electrode. Silicide films are formed on the source/drain regions so as to be separated from the element isolation region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2003-327653, filed Sep. 19, 2003,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including afield-effect transistor using a salicide (self-aligned silicide)structure and a method of fabricating the same.

2. Description of the Related Art

A transistor using a salicide structure is a structure most often usedas a method of reducing the parasitic resistance on a contact junctionor gate electrode (e.g., Jpn. Pat. Appln. KOKAI Publication No.11-8387).

A salicide structure is as follows. As shown in FIG. 1, after a gateelectrode 101 is formed, an insulating film such as an oxide film whichdoes not react with a silicide material is left behind as a gatesidewall film 102 on the side surface of the gate electrode 101. On thisstructure, a metal film for forming a silicide is formed. The siliconand metal are caused to react with each other by heat, thereby formingsilicide films 104 in self-alignment on exposed source/drain regions103.

Recently, the advance of the scaling (downsizing) of devices makes itnecessary to shallow contact junctions (to form shallow junctions).However, it is difficult to scale the silicide reaction layer itself,and this is one cause which interferes with the scaling of source/drainregions. Especially in portions of an element region(active region) nearthe edges of an element isolation region 105 using STI (Shallow TrenchIsolation), it is difficult to control ion implantation or diffusion tothe same depth as a flat portion separated from the edges. Thisdeteriorates the margin to a junction leak on the junction surfacesbetween the source/drain regions 103 and a semiconductor substrate 106when a salicide structure is formed.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the present invention, a semiconductor devicecomprises an element isolation region formed in a semiconductorsubstrate to electrically isolate an element region where an element isto be formed; gate insulating films formed on the semiconductorsubstrate in the element region; a gate electrode formed on the gateinsulating film; source/drain regions formed to be separated from eachother in a surface region of the semiconductor substrate, thesource/drain regions sandwiching a channel region formed below the gateinsulating film; a gate sidewall film formed on two side surfaces of thegate electrode; and silicide films formed on the source/drain regions soas to be separated from the element isolation region.

According to another aspect of the present invention, a semiconductordevice fabrication method comprises forming, in a semiconductorsubstrate, an element isolation region for defining an element regionwhere an element is to be formed; forming a gate insulating film on thesemiconductor substrate in the element region; forming a gate electrodeon the gate insulating film; forming extension regions in a surfaceregion of the semiconductor substrate on two sides of the gateelectrode; forming gate sidewall films on two side surfaces of the gateelectrode; forming source/drain regions in a surface region of thesemiconductor substrate outside the gate sidewall films; forming a metalfilm on the gate electrode, gate sidewall film, and source/drainregions; removing the metal film by anisotropic etching such that themetal films separated from the element isolation region remains only onside surfaces of the gate sidewall films; and silicifying the metalfilms remaining on the source/drain regions to form silicide films,separated from the element isolation region, on the source/drainregions.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a sectional view showing the structure of a conventional MOSfield-effect transistor (to be referred to as a MOSFET hereinafter);

FIG. 2 is a sectional view showing the structure of a MOSFET of thefirst embodiment of the present invention;

FIG. 3 is a plan view of a MOSFET of the first embodiment of the presentinvention;

FIG. 4 is a sectional view showing the first step of a method offabricating the MOSFET of the first embodiment of the present invention;

FIG. 5 is a sectional view showing the second step of the method offabricating the MOSFET of the first embodiment of the present invention;

FIG. 6 is a sectional view showing the third step of the method offabricating the MOSFET of the first embodiment of the present invention;

FIG. 7 is a sectional view showing the fourth step of the method offabricating the MOSFET of the first embodiment of the present invention;

FIG. 8 is a sectional view showing the fifth step of the method offabricating the MOSFET of the first embodiment of the present invention;

FIG. 9 is a sectional view showing the structure of a MOSFET of thesecond embodiment of the present invention;

FIG. 10 is a sectional view showing the first step of a method offabricating the MOSFET of the second embodiment of the presentinvention;

FIG. 11 is a sectional view showing the second step of the method offabricating the MOSFET of the second embodiment of the presentinvention;

FIG. 12 is a sectional view showing the third step of the method offabricating the MOSFET of the second embodiment of the presentinvention;

FIG. 13 is a sectional view showing the fourth step of the method offabricating the MOSFET of the second embodiment of the presentinvention;

FIG. 14 is a sectional view showing the fifth step of the method offabricating the MOSFET of the second embodiment of the presentinvention; and

FIG. 15 is a sectional view showing the sixth step of the method offabricating the MOSFET of the second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below withreference to the accompanying drawing. In the following explanation, thesame reference numerals denote the same parts throughout the drawing.

First Embodiment

First, a MOS field-effect transistor (to be referred to as a MOSFEThereinafter) as a semiconductor device of the first embodiment of thepresent invention will be described below.

FIG. 2 is a sectional view showing the structure of the MOSFET of thefirst embodiment. FIG. 3 is a plan view of the MOSFET. FIG. 2 shows asectional structure taken along a line 2-2 in FIG. 3.

As shown in FIG. 2, an element isolation region 12 and an element regionsurrounded by the element isolation region 12 are formed on a siliconsemiconductor substrate 11. The element region is a region where anelement (in this embodiment, a MOSFET) is to be formed, and iselectrically insulated by the element isolation region 12. A gateinsulating film 13 is formed on the semiconductor substrate 11 in thiselement region, and a gate electrode 14 is formed on the gate insulatingfilm 13.

In addition, extension regions 15 and source/drain regions 16 are soformed as to sandwich a channel region formed on the semiconductorsubstrate 11 below the gate insulating film 13. The extension regions 15are made of shallow diffusion layers having a conductivity type oppositeto that of the semiconductor substrate 11. The source/drain regions 16are made of diffusion layers having a conductivity type opposite to thatof the semiconductor substrate 11 and deeper than the extension regions15. A gate sidewall insulating film 17 is formed on the two sidesurfaces of the gate electrode 14. Furthermore, on the source/drainregions 16 outside the gate sidewall insulating film 17, silicide films18 are formed away from the element isolation region 12.

In this MOSFET as shown in FIG. 2, the source/drain regions 16 near theelement isolation region 12 are formed shallow from the substratesurface. That is, the p-n junction surfaces formed between thesemiconductor substrate 11 and source/drain regions 16 are shallow fromthe substrate surface. In the MOSFET having this structure shown in FIG.2, the silicide films 18 are formed away from the edges of the elementisolation region 12. Since the silicide films 18 are not formed in theshallow portions of the source/drain regions 16 near the elementisolation region 12, a junction leak current produced on the p-njunction surfaces between the semiconductor substrate 11 andsource/drain regions 16 can be reduced. That is, this MOSFET canincrease the junction leak margin in the shallow portions of the contactjunction layers.

This will be explained with reference to the plan view of FIG. 3. Thesilicide layers 18 are formed along the gate sidewall insulating film17, and are not in contact with the element isolation region 12 in thegate longitudinal direction. In other words, in regions in which theelement isolation region 12 and source/drain regions 16 are in contactwith each other and which are parallel to a gate line, the silicidefilms 18 are formed away from the element isolation region 12.Therefore, the silicide films 18 are not formed in those regions nearthe element isolation region 12, in which the p-n junction surfacesbetween the semiconductor substrate 11 and source/drain regions 16 areshallow. Accordingly, the distance between the bottom surfaces of thesilicide films 18 and the p-n junction surfaces can be ensured, and anincrease in junction leak current on the p-n junction surfaces can besuppressed.

In the first embodiment as described above, no salicide film is formednear the edges of an element isolation insulating film; salicide filmsare formed away from those edges of the element isolation insulatingfilm, in which the junction depth of diffusion layers formingsource/drain regions may be small. This makes it possible to wellincrease the distance between the salicide film bottom surfaces and thejunction surfaces (the boundary surfaces between the source/drainregions and substrate). As a consequence, a MOSFET having a largejunction leak margin can be formed.

A method of fabricating the MOSFET of the first embodiment will bedescribed below. FIGS. 4 to 8 are sectional views showing the steps ofthe method of fabricating the MOSFET of the first embodiment.

First, as shown in FIG. 4, an element isolation structure, a gatestructure, a gate sidewall structure, and source/drain diffusion layersare formed by the conventional MOSFET formation process. Details are asfollows. An element isolation region 12 is formed in a siliconsemiconductor substrate 11, thereby forming an element region defined bythe element isolation region 12. The element isolation region 12 isformed by, e.g., STI (Shallow Trench Isolation). The STI is a structurein which an insulating film such as a silicon oxide film is formed in ashallow trench of a substrate. A gate insulating film 13 is formed onthe semiconductor substrate 11 in the element region, and a gateelectrode 14 is formed on the gate insulating film 13. The gateinsulating film 13 is made of, e.g., a silicon oxide film formed bythermal oxidation, and the gate electrode 14 is made of a polysiliconfilm.

Subsequently, the gate electrode 14 is used as a mask to dope animpurity having a conductivity type opposite to that of thesemiconductor substrate 11 by ion implantation, thereby formingextension regions 15 in surface regions of the semiconductor substrate11 on the two sides of the gate electrode 14. In addition, an insulatingfilm such as a silicon oxide film or silicon nitride film is formed onthe structure, and removed by anisotropic etching such as RIE, therebyforming a gate sidewall insulating film 17 on the two side surfaces ofthe gate electrode 14. The gate electrode 14 and gate sidewallinsulating film 17 are used as masks to dope an impurity having theconductivity type opposite to that of the semiconductor substrate 11 byion implantation, thereby forming source/drain regions 16 in the surfaceregions of the semiconductor substrate 11 outside the gate sidewallinsulating film 17. As described earlier, the extension regions 15 aremade of shallow diffusion layers, and the source/drain regions 16 aremade of diffusion layers deeper than the extension regions 15.

As shown in FIG. 5, a metal film 19 for forming a salicide is formed bysputtering or CVD on the structure shown in FIG. 4. Examples of themetal film for forming a salicide are cobalt (Co), nickel (Ni), titanium(Ti), and a stacked film of these metals.

As shown in FIG. 6, the metal film 19 is removed by anisotropic etchingsuch as RIE so as to be left behind only on the two side surfaces of thegate sidewall insulating film 17. The metal film 19 is not in contactwith the element isolation region 12, but is separated from the elementisolation region 12 by a predetermined distance.

Then, the semiconductor substrate having the structure shown in FIG. 6is annealed to form a salicide. Consequently, the silicon forming thesource/drain regions 16 and the element forming the metal film 19 reactwith each other to silicify the metal film 19 on the source/drainregions 16. In this manner, as shown in FIG. 7, silicide films 18 areformed on the source/drain regions 16.

After that, the unreacted portion of the metal film 19 is selectivelyetched away by using, e.g., a hot sulfuric acid/hydrogen peroxideaqueous solution (SH) which is a mixture of sulfuric acid and an aqueoushydrogen peroxide solution. As a consequence, as shown in FIG. 8, thesilicide films 18 are left behind on the source/drain regions 16 so asto be separated from the edges of the element isolation region 12 andextended along the gate sidewall insulating film 17.

In the above fabrication method, after a metal film for forming asilicide is formed on the entire surface, anisotropic etching such asRIE is performed to leave the metal film, separated from the elementisolation region (e.g., STI), only in the vicinities of the gatesidewall film. When salicide formation is performed in the subsequentannealing step, therefore, silicide films can be formed inself-alignment only in the vicinities of the gate sidewall film, and nosilicide film is formed near the element isolation region. Consequently,in the MOSFET formed by the above fabrication method, a junction leakcurrent produced on the p-n junction surfaces of diffusion layersforming the source/drain regions can be reduced.

Second Embodiment

A MOS field-effect transistor (MOSFET) as a semiconductor device of thesecond embodiment of the present invention will be described below.

FIG. 9 is a sectional view showing the structure of the MOSFET of thesecond embodiment. As shown in FIG. 9, a silicide film 18 is formed on agate electrode 14. The rest of the structure is the same as the firstembodiment. In this MOSFET, the wiring resistance in the gate electrodecan be reduced, so the device characteristics can be improved.

A method of fabricating the MOSFET of the second embodiment will beexplained below. FIGS. 10 to 15 are sectional views showing the steps ofthe method of fabricating the MOSFET of the second embodiment.

First, as shown in FIG. 10, an element isolation structure, a gatestructure, a gate sidewall structure, and source/drain diffusion layersare formed by the conventional MOSFET formation process. When the gatestructure is formed, a material with which an etching selective ratio toa gate sidewall insulating film 17 and element isolation region 12 isobtained in the subsequent steps is deposited on a gate electrode 14 andpatterned simultaneously with the gate electrode 14, thereby forming acap film 20. When a silicon oxide film, for example, is to be used asthe gate sidewall insulating film 17 and element isolation region 12, asilicon nitride film is used as the material with which an etchingselective ratio to the gate sidewall insulating film 17 and elementisolation region 12 is obtained. The element isolation region 12, a gateinsulating film 13, extension regions 15, the gate sidewall insulatingfilm 17, and source/drain regions 16 are formed in the same manner as inthe first embodiment.

As shown in FIG. 11, only the cap film 20 on the gate electrode 14 isselectively removed. Subsequently, as shown in FIG. 12, a metal film 19for forming a salicide is formed on the structure shown in FIG. 11 by,e.g., sputtering or CVD. Examples of the metal film for forming asalicide are cobalt (Co), nickel (Ni), titanium (Ti), and a stacked filmof these metals.

As shown in FIG. 13, the metal film 19 is removed by anisotropic etchingsuch as RIE so as to be left behind only on the two side surfaces of thegate sidewall insulating film 17 and on the gate electrode 14. The metalfilm 19 is not in contact with the element isolation region 12, but isseparated from the element isolation region 12 by a predetermineddistance.

Then, the semiconductor substrate having the structure shown in FIG. 13is annealed to form a salicide. Consequently, the silicon forming thesource/drain regions (silicon substrate) 16 and gate electrode(polysilicon film) 14 and the element forming the metal film 19 reactwith each other to silicify the metal film 19 on the source/drainregions 16 and on the gate electrode 14. In this manner, as shown inFIG. 14, silicide films 18 are formed on the source/drain regions 16 andgate electrode 14.

After that, the unreacted portion of the metal film 19 is selectivelyetched away by using, e.g., a hot sulfuric acid/hydrogen peroxideaqueous solution (SH) which is a mixture of sulfuric acid and an aqueoushydrogen peroxide solution. As a consequence, as shown in FIG. 15, thesilicide films 18 are left behind on the source/drain regions 16 so asto be separated from the edges of the element isolation region 12 andextended along the gate sidewall insulating film 17. The silicide film18 is also left behind on the gate electrode 14.

In the above fabrication method, a cap film formed on a gate electrodeis removed after a gate sidewall insulating film is formed on the twoside surfaces of the gate electrode and on the two side surfaces of thecap film. Subsequently, a metal film for forming a silicide is formed onthe entire surface, and anisotropic etching such as RIE is performed.Consequently, the metal film remains in the vicinities of thegate'sidewall film so as to be separated from an element isolationregion (e.g., STI), and also remains on the gate electrode. Whensalicide formation is performed in the subsequent annealing step,therefore, silicide films can be formed in self-alignment only on thesource/drain regions near the gate sidewall film and on the gateelectrode, and no silicide film is formed near the element isolationregion. Consequently, in the MOSFET formed by the above fabricationmethod, a junction leak current produced on the p-n junction surfaces ofdiffusion layers forming the source/drain regions can be reduced. Inaddition, the wiring resistance of the gate electrode can be reduced.

In each embodiment of the present invention as described above, it ispossible to provide a semiconductor device including a field-effecttransistor having a salicide structure in which a large junction leakmargin is obtained because a large distance can be ensured between thebottom surfaces of silicide films and the junction surfaces betweensource/drain regions and a semiconductor substrate, and to provide amethod of fabricating the semiconductor device.

Note that each embodiment described above is of course applicable toboth an n-channel MOS transistor and p-channel MOS transistor.

Note also that the above embodiments can be practiced not only singlybut also in the form of an appropriate combination. Furthermore, theabove embodiments include inventions in various stages, so theseinventions in various stages can also be extracted by appropriatelycombining a plurality of constituent elements disclosed in theembodiments.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit and scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A semiconductor device comprising: an element isolation region formedin a semiconductor substrate to electrically isolate an element regionwhere an element is to be formed; a gate insulating film formed on thesemiconductor substrate in the element region; a gate electrode formedon the gate insulating film; source/drain regions formed to be separatedfrom each other in a surface region of the semiconductor substrate, thesource/drain regions sandwiching a channel region formed below the gateinsulating film; gate sidewall films formed on two side surfaces of thegate electrode; and silicide films formed on the source/drain regions soas to be separated from the element isolation region.
 2. The deviceaccording to claim 1, wherein the silicide films are formed in contactwith side surfaces of the gate sidewall films.
 3. The device accordingto claim 1, wherein the silicide films contain one of cobalt (Co),nickel (Ni), and titanium (Ti).
 4. The device according to claim 1,wherein the element isolation region is made of STI (Shallow TrenchIsolation) formed by burying an insulating film in a trench formed inthe semiconductor substrate.
 5. A semiconductor device comprising: anelement isolation region formed in a semiconductor substrate toelectrically isolate an element region where an element is to be formed;a gate insulating film formed on the semiconductor substrate in theelement region; a gate electrode formed on the gate insulating film; afirst silicide film formed on the gate electrode; source/drain regionsformed to be separated from each other in a surface region of thesemiconductor substrate, the source/drain regions sandwiching a channelregion formed below the gate insulating film; gate sidewall films formedon two side surfaces of the gate electrode and on two side surfaces ofthe first silicide film; and second silicide films formed on thesource/drain regions so as to be separated from the element isolationregion.
 6. The device according to claim 5, wherein the first silicidefilm is formed in contact with side surfaces of the gate sidewall films.7. A semiconductor device fabrication method comprising: forming, in asemiconductor substrate, an element isolation region for defining anelement region where an element is to be formed; forming a gateinsulating film on the semiconductor substrate in the element region;forming a gate electrode on the gate insulating film; forming extensionregions in a surface region of the semiconductor substrate on two sidesof the gate electrode; forming gate sidewall films on two side surfacesof the gate electrode; forming source/drain regions in a surface regionof the semiconductor substrate outside the gate sidewall films; forminga metal film on the gate electrode, gate sidewall films, andsource/drain regions; removing the metal film by anisotropic etchingsuch that the metal films separated from the element isolation regionremains only on side surfaces of the gate sidewall films; andsilicifying the metal films remaining on the source/drain regions toform silicide films, separated from the element isolation region, on thesource/drain regions.
 8. The method according to claim 7, wherein thesilicide films are formed along the gate sidewall films inself-alignment with the source/drain regions.
 9. The method according toclaim 7, wherein the silicide films are formed in contact with sidesurfaces of the gate sidewall films.
 10. The method according to claim7, wherein forming the gate sidewall films comprise forming aninsulating film on the gate electrode and semiconductor substrate, andremoving the insulating film by anisotropic etching to leave theinsulating film behind on the two side surfaces of the gate electrode.11. The method according to claim 7, wherein forming the extensionregions comprise doping an impurity having a conductivity type oppositeto that of the semiconductor substrate by ion implantation by using thegate electrode as a mask, thereby forming a diffusion layer.
 12. Themethod according to claim 7, wherein forming the source/drain regionscomprise doping an impurity having a conductivity type opposite to thatof the semiconductor substrate by ion implantation by using the gateelectrode and the gate sidewall films as masks, thereby forming adiffusion layer deeper than the extension regions from the surface ofthe substrate.
 13. The method according to claim 7, further comprisingremoving the metal film which is not silicified but kept unreacted,after forming the silicide films.
 14. The method according to claim 7,wherein the element isolation region is made of STI (Shallow TrenchIsolation) formed by burying an insulating film in a trench formed inthe semiconductor substrate.
 15. The method according to claim 7,wherein the metal film is made of at least one of cobalt (Co), nickel(Ni), and titanium (Ti).
 16. A semiconductor device fabrication methodcomprising: forming, in a semiconductor substrate, an element isolationregion for defining an element region where an element is to be formed;forming a gate insulating film on the semiconductor substrate in theelement region; forming a gate electrode on the gate insulating film,and a cap film on the gate electrode; forming extension regions in asurface region of the semiconductor substrate on two sides of the gateelectrode; forming gate sidewall films on two side surfaces of the gateelectrode and on two side surfaces of the cap film; forming source/drainregions in a surface region of the semiconductor substrate outside thegate sidewall films; removing the cap film on the gate electrode;forming a metal film on the gate electrode, gate sidewalls film, andsource/drain regions; removing the metal film by anisotropic etchingsuch that the metal films separated from the element isolation regionremains on side surfaces of the gate sidewall films, and that the metalfilm remains on the gate electrode; and silicifying the metal filmsremaining on the source/drain regions and gate electrode to form firstsilicide films, separated from the element isolation region, on thesource/drain regions, and a second silicide film on the gate electrode.17. The method according to claim 16, wherein the first silicide filmsare formed along the gate sidewall films in self-alignment with thesource/drain regions.
 18. A method according to claim 16, wherein thefirst silicide films are formed in contact with the side surfaces of thegate sidewall films.
 19. The method according to claim 16, whereinforming the gate sidewall films comprise forming an insulating film onthe cap film, gate electrode, and semiconductor substrate, and removingthe insulating film by anisotropic etching to leave the insulating filmbehind on the two side surfaces of the gate electrode and on the twoside surfaces of the cap film.
 20. The method according to claim 16,wherein the cap film is made of a material with which an etchingselective ratio to the gate sidewall films and element isolation regionis obtained.